1. Field of the Invention
The present invention relates generally to automatic exposure adjusting apparatuses, and more particularly, to an automatic exposure adjusting apparatus contained in such an image sensing apparatus as a video camera for, when exposure is automatically adjusted based on a luminance evaluating value detected from a video signal obtained from an image sensor, subjecting a priority processing to the luminance evaluating value in accordance with a luminance distribution in an image sensed picture by fuzzy inference.
2. Description of the Background Art
Conventionally, an image sensing apparatus such as a video camera having an apparatus for automatically adjusting exposure or luminance level in response to a video signal obtained from an image sensor has been put into practice. As such conventional automatic exposure adjusting systems, there are two types of such systems, that is; a system of detecting such levels as a mean value and a peak value of a luminance level of a video signal obtained from an image sensor and mechanically adjusting an optical iris of a lens based on the detected levels and a system of electrically adjusting exposure by adjusting a gain of an amplifier for amplifying a level of a video signal obtained from an image sensor.
However, such a conventional automatic exposure adjusting apparatus has the following disadvantages. For example, when a high luminance portion such as a light source is included in a picture, a gain of the entire picture is decreased, so that a major object becomes insufficiently bright. On the contrary, when the background is very dark, the gain of the entire picture is increased, whereby the major object becomes too bright.
An exposure correcting method for eliminating such disadvantages has been proposed in, for example, Japanese Patent Laying-Open No. 62-110369. In this exposure correcting method, in view of the fact that a major object is arranged in the center of the picture in many cases, the picture is divided into a central region and a peripheral region, from each of which a luminance level of a video signal is detected as an evaluating value. Then, adjustment of the exposure is made based on a ratio of an evaluating value of the peripheral region to that of the central region to obtain an optimum exposure for the major object positioned in the central region of the picture.
FIG. 1 is a block diagram of an automatic exposure adjusting apparatus using such conventional method.
Light from an object, after passing through a lens 1 and having its amount adjusted by an optical iris mechanism 2, enters an image sensing circuit 3. The image sensing circuit 3 photoelectrically-converts the incident light and outputs the photoelectrically-converted light as a video signal.
The obtained video signal is amplified by a variable gain amplifier 4 to be sent to a not-shown video circuit and is also compared by a comparator 5 with a desired luminance level which will be described later. An iris motor 6 is driven in response to a difference voltage generated from the comparator 5, whereby the size of the aperture of the iris mechanism 2 is controlled.
The video signal outputted from the image sensing circuit 3 is also sent to a region selecting circuit 19, which circuit supplies a video signal in a priority region to an integrating circuit 20 comprising a digital integrator for detecting a level and supplies a video signal in a non-priority region to an integrating circuit 21 similarly comprising a digital integrator in response to a switching signal for separating regions obtained by a synchronization separating circuit 12 and a switching control circuit 18. Each of which integrating circuits 20 and 21 integrates one field of the supplied video signal.
Outputs of the integrating circuits 20 and 21 are applied to a dividing circuit 15, which circuit generates a signal corresponding to a ratio of both outputs and applies the signal to a gain controlling circuit 16 and a target luminance level controlling circuit 17. Both the controlling circuits 16 and 17 correct exposure by changing the gain of the variable gain amplifier 4 and a target luminance level of the optical iris based on the result obtained from the dividing circuit 15.
In such method of giving the central region a priority, a manner of setting a priority region is important. For example, if the priority region is set to be small, a luminance level of the priority region fluctuates greatly, so that if the exposure is corrected in response thereto, a luminance level of the entire picture greatly fluctuates. On the contrary, if the priority region is set to be large, while the luminance level of the entire picture becomes stable, the priority region includes not only the major object but also the background depending on the location, configuration of the major object and the like, whereby correction of the exposure is not sufficiently made.
One solution to this problem is to subdivide the priority region and make a priority of each region variable to perform a fine priority processing corresponding to various pictures. However, an image sensed picture has various luminance distributions in practice, so that it is difficult to set a value by classifying all the cases based on numerical conditions because of strict restrictions of a capacity of the system and processing capability.
In addition, in case such an extremely high luminance object as a light source is included in the priority region or it occupies a large area of the picture, fixing the priority region at the center of the picture as the above-described conventional art allows neighboring major objects to become dark due to a reverse effect of the correction or a lack of the amount of the correction.
Generally, in a picture of a low luminance such as at night or in a dark room, a difference in luminance levels between the regions mainly depends on a reflection factor of the object itself such as black and white rather than the brightness. If the same correction is made to such a picture as that is made to the image sensed picture having a large difference in brightness outdoors as the above-described conventional art, unnecessarily intensive correction is made for the difference in the reflection factor of the object itself such as black or white, so that a unnatural picture is produced.
On the other hand, in the automatic exposure adjusting apparatus shown in FIG. 1, a ratio of the output of the integrator 20 to that of the integrator 21 becomes 1:3 in a rear light state, that is, in a state wherein the light source is included in the non-priority region, a luminance level of the priority region is, for example, 1/3 times that of the non-priority region and the areas of both the regions are the same, so that an optimum brightness can be obtained for the object in the priority region by setting a target luminance level to be higher than usual.
Such conventional technique is effective for correcting exposure in a so-called rear light state wherein an abnormally high luminance portion such as a light source is included in the non-priority region, making the background in the non-priority region remarkably brighter than the major object in the priority region, the technique might adversely affect the picture in a so-called excessive follow light state wherein the major object is remarkably brighter than the background. Namely, generally in the excessive follow light state, if the luminance level is corrected to the same level as in the rear light state, the peripheral portion of the picture wherein the background exists becomes dark to make the picture have an impression different from that received from the actual scene.
The solution to these problems is proposed and described in the copending U.S. patent application Ser. No. 294,866 filed on the Jan. 9, 1989 and assigned to the same assignee of the instant application.
The first of these solutions is computing a luminance evaluating value indicating a luminance level of each region by dividing an image sensed picture into a plurality of regions in advance, extracting a video signal in each region and integrating one field of a low frequency component of the signal. Then, the luminance evaluating value of each region is compared with a reference value which is expected to be obtained when such an abnormal luminance portion as a light source is included in the region and a determination is made that a region having a luminance evaluating value exceeding the reference value includes an abnormal luminance portion. Then, by controlling exposure such that a mean value of the luminance evaluating values in the regions excluding the region including the abnormal luminance portion coincides with the target value, the influence of the abnormal luminance portion on the entire image sensed picture is eliminated, so that an optimum exposure for the object in the regions without the abnormal luminance portion is obtained.
The second solution is designating as a priority region a central region of the picture expected to have a particularly high probability of including the major object among the above-described plurality of regions, weighting a luminance level of the priority region and then computing an average luminance evaluating value (representative value) with respect to all regions. Then, by controlling exposure so as to make the representative value coincide with the target value, an optimum exposure for the major object can be obtained.
The third solution, while in a normal state of picture taking the above-described second solution is carried out, is always monitoring the luminance level of each region becoming remarkably high or low, that is, an abnormally high luminance portion or an abnormally low luminance portion being included in any of the regions, so that if there is a region including the abnormal luminance portion, a luminance level of this region is prevented from affecting the computation of the above-described representative value, thereby preventing the effect of such abnormal luminance portion as a light source on the entire picture.
Furthermore, the fourth counter measure is, in the normal state of taking picture, giving a central region of the picture a priority as a major region over the other regions to adjust exposure such that a luminance level of the major region becomes an optimum level and when the major region includes such an abnormally high luminance portion as a light source or such an abnormally low luminance portion as dark green, controlling exposure such that an average luminance level of the other regions except for the major region becomes an optimum level.
As the above-described first counter measure, it is very effective for taking picture in a rear light or an excessive follow light state to divide the image sensed picture and detect the existence of an abnormal luminance portion or a location of the same on the picture, that is, detect a so-called luminance distribution in the image sensed picture and make the adjustment of exposure corresponding to the detection result.
However, it is impossible to distinguish a state wherein a luminance evaluating value slightly exceeds the reference value from a state wherein it largely exceeds the same by a method of simply dividing the cases depending on whether the luminance evaluating value exceeds the reference value or not in detecting a luminance distribution as the first method. Accordingly, for example, in case a luminance evaluating value of the region including the abnormal luminance portion is approximately equal to the reference value, the luminance evaluating value becomes higher than the reference value at some time points and becomes lower at other time points, so that exposure controlling is made every time, of holding the regions without the abnormal luminance portion in an optimum exposure state with the influence of the abnormal luminance portion being disregarded and holding only the region including the abnormal luminance portion in an optimum exposure state responsive to the influence of the abnormal luminance portion, whereby the brightness of the entire picture intermittently varies in response thereto to make the picture unsightly.
In addition, if the light source which is of a low luminance in the initial state gradually becomes a high luminance, a luminance evaluating value of the region including the light source in due course exceeds the reference value. Then, since the effect of the light source on the adjustment of exposure largely varies immediately before and after the luminance evaluating value exceeds the reference value, the brightness of the image sensed picture (the region without the light source) abruptly changes from dark to bright at the turning point at which the luminance evaluating value exceeds the reference value, resulting in a unsightly picture.
Now, considered is a case wherein the picture is divided into six regions of A1 through A6 as shown in FIG. 3 and the central regions A1 and A2 are considered to be priority regions in the above-described second counter measure. In this case, assuming that a light source such as sun enters the region A2 in taking a picture and a major object S is located in the regions A1 and A3 as indicated by the oblique lines as in FIG. 5, the major object S is in a rear light state, so that luminance levels of the regions A1 and A3 become approximately the same low levels and a luminance level of the region A2 becomes extremely high.
Under such circumferences, if the priority regions A1 and A2 are weighted as described above, the representative value of the picture becomes high due to the influence of the luminance level of the region A2. Accordingly, if exposure is adjusted based on these luminance levels, the major object becomes extremely lacking in exposure.
On the contrary, if such an abnormally low luminance portion as dark green enters the region A2, the major object is extremely oversupplied with exposure.
Now, while the above-described third countermeasure is very effective in a rear light or an excessive follow light state, the following problems arise in intentionally taking a picture of such an abnormal luminance portion as a light source. Namely, considering, for example, a case wherein a light source is taken picture of the image sensing apparatus is logically fixed to a position such that the light source is situated in a major region at the center of the picture. On this occasion, since a luminance level of the major region becomes remarkably high, it can not concern the calculation of a representative value, so that exposure is adjusted such that the regions excluding the major region enter an optimum exposure state. As a result, the major region becomes highly luminant to make taking a picture of the light source itself difficult.
In the above-described fourth countermeasure, in a normal picture taking, a region wherein the major object is highly probably included is designated as the central region of the picture and the other regions are uniformly designated as regions wherein the major object is less probably included. However, in actually taking a picture, the other regions than the central region of the picture slightly differ in probability of the existence of the major object, so that the above-described fourth countermeasure has a problem that a fine adjustment of exposure can not be made corresponding to these subtle differences.